Conventionally, in a thin-film solar cell, there is used a tandem structure having plural photoelectric conversion layers (semiconductor layers) made of materials of different bandgaps stacked on a translucent insulating substrate so as to widely and effectively use a solar spectrum. Particularly, in a case of thin-film solar cells of a silicon series, a structure having amorphous silicon cells and microcrystalline silicon cells stacked as a semiconductor layer is employed in many cases. Each of the cells has a three-layer structure having a P-type semiconductor film, an I-type semiconductor film, and an N-type semiconductor film sequentially stacked thereon. The I-type semiconductor film is an electric-power generation layer, and the P-type semiconductor film and the N-type semiconductor film are layers for forming an internal electric field.
Generally, in microcrystalline silicon cells, a film including many microcrystalline-silicon crystalline grains that are oriented in the (220) plane is preferable as the I-type semiconductor film of an electric-power generation layer, and crystalline grains that are oriented in the (111) plane are regarded unsuitable. This is because grain boundaries of microcrystalline silicon crystals that are oriented in the (111) plane easily adsorb impurities such as oxygen, carbon, and nitrogen from open air. When these impurities are adsorbed, a film that includes these crystalline grains functions as a leak current path. Consequently, when this film is applied to a thin-film solar cell, power generation efficiency is considerably lowered.
However, it has been known that when a microcrystalline-silicon thin film is formed by a chemical vapor deposition (CVD) method or the like, crystalline grains that are oriented in the (111) plane are more stable and grow in higher priority than crystalline grains that are oriented in the (220) plane. Over several decades, many enterprises and research organizations in the world have hitherto made researches and developments concerning a film forming processing technique to grow a microcrystalline silicon film that is perfectly oriented in the (220) plane without including the (111) plane orientation. However, no effective solution has been found yet.
Therefore, according to a conventional method for manufacturing a microcrystalline silicon film for a thin-film solar cell, the following examination has been made. That is, a plasma is generated by introducing hydrogen gas into a film forming chamber after a microcrystalline silicon film is formed. Crystalline grain boundaries in the microcrystalline silicon film are passivated by hydrogen radicals (H) that are generated by plasma dissociation, thereby reducing adsorption of impurities (see, for example, Patent Literature 1).
The following examination has been also made. That is, a radical generation chamber is provided at a side of a film forming chamber of a microcrystalline silicon film, and after a microcrystalline silicon film is formed, hydrogen radicals (H) generated in the radical generation chamber are introduced into the film forming chamber. Grain boundaries of microcrystalline silicon that are oriented in the (111) plane are passivated by the hydrogen radicals (H), thereby reducing adsorption of impurities (see, for example, Patent Literature 2).